In this project we have several aims: Aim 1: To integrate information obtained by functional biochemical studies on ectopically expressed non-structural HTLV-I proteins to infectivity and persistence of HTLV-I in relevant cellular models in vitro (dendritic cells and T-cells) Aim 2: To integrate information obtained by functional biochemical studies on ectopically expressed non-structural HTLV-I proteins to infectivity and persistence of HTLV-I in relevant animal models of HTLV-I infection. The viral genome encodes mRNAs for several non-structural proteins that affects cellular pathways and modulate viral replication in vitro. The p12 protein, encoded by orf I, localizes to the ER and Golgi and cellular membranes and different p12I cellular localization and functions are dictated by proteolytic cleavage. The removal of a non-canonical endoplasmic reticulum (ER) retention/retrieval signal within the amino terminus of p12I is necessary for trafficking to the Golgi apparatus and the generation of a completely cleaved 8 kDa protein. The 8 kDa protein in turn traffics to the cell surface, is recruited to the immunological synapse following T-cell receptor (TCR) ligation and down-regulates TCR proximal signaling. The uncleaved form of p12I resides in the ER and interacts with the b and gc chains of the interleukin-2 receptor (IL-2R), the heavy chain of the major histocompatibility complex (MHC) class I, as well as calreticulin and calnexin. Genetic analysis of ORF-I from ex vivo samples of HTLV-1-infected patients reveals predominant amino acid substitutions within ORF-I that inhibits proteolytic cleavage, suggesting that ER associated functions of p12I may be selected in vivo. We plan to use reverse genetic and use animal models to understand the contribution of variants of this viral gene to the maintenance of viral load in the host. The HTLV-I orf II encodes p30II, a nuclear/nucleolar protein that not only regulates viral expression by a post-transcriptional mechanism, but also affects the expression of genes involved in host responses such as TLR-4. In addition Orf II encodes p13, a small protein that we have recently demonstrate to decrease viral replication by targeting and degrading Tax, the viral trans activator. We have generated genetic mutant of HTLV-I that do not express p13 and/or p30 and found that the p30 affects dramatically the ability of HTLV-I to replicate in monocytoid-derived dendritic cells. The effect of p30 and p13 on in vitro infectivity is being tested in animal models. We also created HTLV-I mutants in the HBZ and found that the lack of HBZ result in an increased HTLV-I fusogenic ability.